The present invention relates to lead-free tin alloys for solder joints with improved mechanical properties, such as creep strength, and improved thermal fatigue strength, and to lead-free solders based on such alloys.
Solder joints are critical in many electronic devices. Furthermore, there is an increasing demand from the automotive industry for electronic devices with improved thermal fatigue strength at increasingly higher operating temperatures.
Many solders are based on tin-lead alloys, but the toxicity of lead in industrial processes, products and as waste at the end of product life, is of great environmental and health concern.
DE-A-2713196 and DE-A-3147226 disclose the use of zinc-aluminium-magnesium alloys as grain refiners in cast tin-based pewter and bearing alloys. The tin alloys do not contain indium and are not alloys for solder joints.
EP-A-0702095 discloses an alloy for a slide bearing which comprises Sn, Cu, Sb, Ni, In and Eutinal, Ag or Ti. The alloy generally comprises from 0.3 to 2% by weight of indium, with an amount of 1.5% by weight of indium being particularly preferred for use.
Lead-free alloys have been developed. For example a tin/silver alloy with respective weight percentages of 96.5% tin and 3.5% silver is known for reflow soldering.
A further example of a lead-free alloy, used for wave soldering, is a eutectic tin/copper alloy consisting of 99.3 wt % tin and 0.7 wt % copper, respectively.
A still further example of a lead-free alloy, used for soldering, is an alloy consisting of 96.2 wt % tin, 2.5 wt % silver, 0.8 wt % copper and 0.5 wt % antimony.
Tin-lead solder alloys have widespread applications for example as ball grid arrays in mounting silicon chips on printed circuit boards. These ball-grid arrays must have sufficient thermal fatigue strength in order to be able to withstand a temperature difference between the component and the circuit board of up to 150.degree. C. over a width of about 4 cm, during turning the circuit on and off. If the thermal fatigue strength of the solder joints is not sufficient, there is a coarsening of the micro-structure of the solder. Subsequently crack initiation and crack propagation occur, which lead in turn to loss of electrical contact between the chip and the printed circuit board.
Accordingly, a problem with the known lead-free tin solder alloys is their thermal fatigue strength.